Re: [Discuss-gnuradio] spectrum repeats at 64MHz

[Marcus D. Leech]

On 12/07/2018 06:01 PM, cesar wrote:
> Hello
>
> Im new in GNUradio and I am trying to build a spectrum analyzer using 
> a USRP, to sweep the spectrum from 0 to 112 MHz. In order to scan the 
> total 112 MHz window, I see individual 8 MHz bandwidth windows at a 
> time. But I see that at 64 MHz the spectrum repeats.
>
> My question is why the spectrum repeats at 64 MHz? and how can I solve 
> it?
>
> Somebody can help me please?
>
> The features of the USRP are:
>
> ADC sample rate = 64 MS/s
> clock frequency = 64 MHz
> daughterboard = BasicRX with range 1-250 MHz
>
> I attach the spectrum of the 0-8 MHz and 64-72 MHz windows to example 
> the case and my flowgraph in GRC.
This presumably is a USRP1, given the 64Msps sample rate.

The Basic_RX card is just a "buffer" between the RF connectors and the 
ADC.  While it has 250Mhz of *analog* bandwidth, you're still subject
  to Nyquist, which means that everyone above 32Mhz will get aliased, 
given the 64Msps ADC.

When using the Basic_RX, particularly as a beginner, it's best to put a 
stiff 30Mhz low-pass filter in front of it.  Once you're comfortable 
with that,
  you can investigate "bandpass sampling" or "sampling in the 2nd or 
3rd Nyquist Zone".